1. Field of the Invention
The present invention relates to aircraft propulsion systems and, more particularly, to a vertical take-off and landing (VTOL) propulsion system for aircraft that employs a distributed system of lifting propellers to achieve vertical take-off and landing as well as highly efficient forward flight.
2. Description of the Background
There are a variety of existing vertical take-off and landing (“VTOL”) aircraft in use today. For example, helicopters are VTOL aircraft. However, because of its retreating blade and its basic construction the forward flight speed and efficiency of a conventional helicopter is significantly inferior to that of a conventional fixed wing aircraft. Additionally, the complexity of the helicopter's mechanical linkages contributes significantly to the crafts high cost and demanding maintenance requirements.
More recent efforts to improve the forward flight speed of vertical take-off and land aircraft are geared toward articulating rotors and/or wings or other toward other means of vectoring thrust. The V-22 Osprey is a twin rotor helicopter with rotors that tilt forward. The Harrier AV 8A accomplishes vertical and horizontal thrust by articulating movable nozzles, which are used to vector the thrust from its turbofan engines. The Lockheed Martin Joint Strike Fighter (JSF) concept is described by Bevilaqua and Shumpert in U.S. Pat. No. 5,209,428 dated May 11, 1993. The Lockheed Martin JSF has a 3-bearing swivel duct, a variable nozzle, and lift fan. The Boeing JSF concept is described by Burnham et al. in U.S. Pat. No. 5,897,078 dated Apr. 27, 1999. This aircraft has rotational lift nozzles near the center, and yaw, pitch and roll nozzles that stabilize the aircraft in a hover. The aircraft uses an F-119 derivative engine, positioned near the air intake. Other efforts include Bollinger in U.S. Pat. No. 5,275,306 dated Jan. 4, 1994, who describes an aircraft with a horizontal lift fan driven by exhaust air. Zimmerman in U.S. Pat. No. 3,972,490 dated Aug. 3, 1994 describes a tri-fan powered VTOL aircraft that uses turbo-fans and has a horizontal lift fan in the nose of the aircraft. Other examples of this type of aircraft include the Bell XV-3, the Curtis-Wright X-100 and the Curtis-Wright X-19, U.S. Pat. No. 6,343,768 by Muldoon, U.S. Pat. No. 5,839,691 by Lariviere, and U.S. Pat. Application Pub. No. 2003/0080242 by Kawai.
The tilt rotor aircraft designs mentioned above attempt to combine the forward flight dynamics of a fixed wing aircraft with the vertical take off and land capabilities of a helicopter. However, tilt rotor aircraft have several distinctive drawbacks. The first notable drawback is that tilt rotor aircraft must overcome negative angular moments created by tilting their spinning rotors 90 degrees during VTOL transitions. These angular moments produce a nose up force when transitioning from vertical to horizontal flight and a nose down force when transitioning from horizontal to vertical flight. These forces create inherently unstable conditions during the transitions between vertical and horizontal flight and visa versa. In actual practice, this inherent instability has been largely responsible for a poor safety record for this type of aircraft. A second drawback of the tilt rotor design is the fact that if the propulsion rotation system should fail the craft is rendered incapable of landing as a conventional fixed wing aircraft. This occurs because the rotors are so large that they would strike the ground if the aircraft were to be landed like a conventional fixed wing aircraft, with the propellers spinning on a horizontal axis. Ducted fan aircraft such as the X-22, the NORD 500 and the Doak 16 reduce the swept area of the propulsion system and allow the aircraft to take off and land horizontally. However, in forward flight the ducted fans build up a boundary layer of air immediately in front of the ducts. This limits their forward speed roughly to that of helicopters.
In U.S. Pat. No. 5,178,344 Dlouhy describes a craft with exposed rotors around the periphery of the aircraft. This craft has a high drag profile and is unsuited for high efficiency forward flight.
High velocity vectored thrust aircraft like the previously mentioned Harrier jet as well aircraft like those shown in U.S. Pat. No. 5,115,996 by Moller, U.S. Pat. No. 4,071,207 by Piasecki, and U.S. Pat. Application Pub. No. 20030062443 by Wagner et al. (specifically their configuration show in FIG. 12) all suffer from at least four major drawbacks. Since vertical and horizontal thrust are controlled by vectoring a common air stream the vertical thrust and horizontal thrust cannot be controlled independently. This interdependence causes serious control and stability issues during VTOL transitions. Secondly, since thrust is gained by vectoring high velocity air, the high velocity air stream will kick up any loose objects in its immediate proximity during take off and landing. This phenomenon can pose a hazard to the aircraft and to ground personal. Thirdly, since high velocity air is used to generate the lifting thrust, more power is required for vertical take off and landing than would be for an aircraft that generates its thrust over a larger area with a slower velocity air stream (i.e. a helicopter). Because of this, not only must the aircraft power plant be capable of supplying the required additional power, but the large amount of fuel used during take off and land negatively affects the aircraft's effective range and flight time. Fourthly, vectored thrust aircraft whose thrust jets are located in close proximity to one another do not provide a wide and stable “base” for the aircraft to balance on and are inherently unstable in hover.
Another type of fixed wing VTOL aircraft is the tail setter. Tail setters rest on their tails and take off and land vertically, rotating the entire craft by 90 degrees to enter and exit forward flight. As with the tilt rotor aircraft, the lack of aerodynamic lift and the negative angular moment caused by tilting the craft with its spinning rotors causes significant instability issues when transitioning between vertical and horizontal flight (and visa versa). Additionally, for piloted aircraft, the tail setter provides the pilot with limited situational awareness during VTOL transitions and hover. Examples of tail setter aircraft include, U.S. Pat. Application Pub. No. 2002/0074452 by Ingram, U.S. Pat. No. 5,863,013 by Schmittle, U.S. Pat. No. 5,758,844 by Cummings, U.S. Pat. No. 5,086,993 by Wainfan, U.S. Pat. Application Pub. No. 2002/0074452 by Ingram, and U.S. Pat. Application Pub. No. 2003/0006339 by Capanna.
Still another type of fixed wing VTOL aircraft employs vertically oriented ducted fans or jets in the in the wing of the craft. This type of aircraft typically suffers from several significant drawbacks. First, if the craft has only a few small fans, high velocity air is required for sufficient thrust thus resulting in the hazards and inefficiencies previously noted for the vectored thrust aircraft. If, however, the fan area is large the area taken by the fans will significantly impair the ability of the wing to develop lift during the transition time, when maximum lift is most needed. Furthermore, if the openings are large, they must be shuttered with louvers in order to reduce the induced drag of the opening during forward flight. This requirement for shuttering the fans during VTOL transitions adds further complexities and instabilities to the aircraft, particularly when transitioning from vertical to horizontal flight and visa-versa. A second major drawback of the fan-in-wing aircraft is that the wings must be thicker than normal in order to house the ducted fans and their associated power transmission or power generation components. The drag induced by the thicker wing geometry will limit forward flight speed and efficiency.
U.S. Pat. No. 5,890,441 by Swinson et al. discloses a design with large ducted fans positioned in the body of the aircraft, not in the wing. However, due to the large vertical openings and the wide fuselage body, the craft will have a large amount of induced drag and therefore is not suited for high efficiency forward flight. Furthermore, the mechanical linkage complexities required to control the aircraft rival that of a helicopter, thus bringing with them the same maintenance and reliability issues found with the helicopter.
There are also non-winged versions of the vertical ducted fan concept, such as U.S. Pat. No. 6,568,630 by Yoeli, U.S. Pat. No. 5,653,404 by Ploshkin, and U.S. Pat. No. 6,179,247 by Milde. Since these non-winged craft derived the majority of their lifting force from vertical thrust, they are inherently inefficient in regards to forward flight when compared to a conventional fixed wing aircraft.
A major drawback of nearly all of the foregoing tilt rotor and tilt-duct designs is that the aircraft is unable to fly at all if one engine should fail. Moreover, the complexity and costliness of such aircraft have been extreme. The aviation industry has long sought to improve these existing tilt-rotor and tilt-duct designs, most importantly improving reliability and safety, speed and range, and reducing or eliminating the risk of stalling. To date the foregoing and all other known attempts have fallen short of at least one of these goals.